Method of forming an oxide coating on tin



United States Patent METHOD OF FORMING AN OXIDE COATING ON TIN John James Russell, Des Plaines, and Herbert Nestor Headland, Chicago, Ill., assignors to Ekco Products Company, a corporation of Illinois No Drawing. Application February 23, 1954, Serial No. 412,112

The portion of the term of the patent subsequent to August 31, 1971, has been disclaimed 4 Claims. (Cl. 204-37) This invention relates to the anodic chemical treatment of tin and tin alloys containing a high percentage of tin and articles made therefrom.

An object of our invention is the provision of a simple, direct and thoroughly practical process for chemically treating tin and tin alloys by anodic treatment in various electrolytes.

Another object of our invention is to provide a process for anodically treating the tin and tin alloy surfaces of articles, such as tin plate, wherein these surfaces shall be characterized by excellent heat absorption, high abrasion resistance, and continuity and uniformity of the treated tin surfaces. U

Another object of our invention is the provision that the anodic heat absorbing film produced by the chemical process for treating the surface of tin or tin alloy, shall be adherent and sufliciently ductile in order that articles may be further fabricated by mechanical drawing, forrning and/ or bending operations.

Another object of our invention is the provision of chemically processing and anodically treating the surface of tin and tin alloy wherein the process is effective over a substantial range of temperature, electrolyte composition and concentration and current density so as to produce an effective anodic film on various types and grades of products having surfaces of tin and tin alloy. Other objects in part will be obvious and in part will be pointed out hereinafter.

As conducive to a clearer understanding of certain features of the invention, it may be noted at this point that tin articles having surfaces of tin or tin alloys shall also include tin plate. Hot dipped tin plate is generally considered to be steel base metal coated on its exterior surface with metallic tin, wherein intermediate to the tin and steel interface, an iron-tin alloy composition is formed. Tin plate may be formed by either the conventional method of hot dipping or electrolytically depositing the tin on the surface.

The coating weight of tin is usually specified as pounds of tin per basis box, or as grams of tin persquare metre of tin plate. The conversion of weight per basis box to linear thickness depends on an assumed density which is compensated by the fact that in the case of hot dipped tin coatings part of the coating is in the form of the tin-iron alloy component.

It is generally accepted that one pound per basis box is equivalent to 0.0000606 inch thick of tin on each face of the tin plate. One and one half pound tin plate is generally assumed to be about 0.0000909 inch thick. The proportional ratio between thickness and total Weight of tin plate can be approximated from the relative proportions above mentioned, for hot dipped tin plate.

In the case of electrolytic tin plate thickness coating of 8 oz. to 10 oz. per basis box is generally acceptable; although some applications of tin plate may use less than one and one-quarter pound but not less than one-half pound per basis box.

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, 2 In the present invention, the anodic chemical treatment of the tin plate is primarily directed toward tin plate having tin, greater than one and one-quarter pound per basis box which would primarily be adaptable to A hot dipped tin plate; although the scope of the invention is also applicable to electrolytic tin platedtarticles.

The formation of oxide films of tin, as heretofore known, may be produced by converting tin into the oxide form by subjecting the tin under oxidizing conditions, such as air, at elevated temperatures. This process of subjecting tin to oxidizing conditions at elevated temperaturehas been conventionally used by the baking industry in converting the tin surfaces into oxides of tin by placing formed articles of tin plate in baking ovens at an elevated temperature of approximately 400 to 425 F. for a period of time of approximately 4 to 12 hours. This has been conventionally called the burning in or burning out process, wherein the baking pans acquire a color range from the interference films of light iridescent hues ranging in color from yellow, blue through greens, and subsequently a film having a degree of opacity may be obtained depending upon the chemical and physical characteristics of the tin plate surface in conjunction with variable atmospheric conditions during the burning in cycle.

It has been found that these oxide films possess wide variations in color characteristics; and it is highly desirable to eliminate these wide variations in order to produce bread of uniform and consistent crust color. In addition, the tiniron alloy layer intermediate to the tin layer and the steel basis is brittle and less corrosion resistant than is the pure tin. Normally, the tin-iron alloy is increased with respect to the available tin layer by progressive burning in operation; and it is highly desirous to reduce the burning in time to about one hour. The baking industry finds it economically undesirable. to subject the bread pans to a long period of burning in; because it ties up the baking ovens, baking pans, as well as the personnel in a non-production operation, and also provides variable results. Also uniformity of crust color of bread is a definite sales factor; and a baker who can produce a uniform loaf of bread in the initial baking operation with a new baking pan set will increase his production capacity.

In addition, the adherence of oxides of tin by the conventional burning in method has been found to be very poor. Chemical as well as atmospheric factors which control the adherence of the oxides of tin are not well defined; and it is known that variations in the chemical and physical composition of the surface of the tin will produce variations in the adherence of the oxide film. Therefore, it is desirable to secure. oxide film formation having heat absorption which is uniform, continuous and tenaciously adherent as well as reproducible and at the same time to have a surface oxide film that will not tend to reflect the heat energy that impinges upon the surface of objects made of tin plate.

An outstanding object of the invention, accordingly, is the provision of an economical industrially practical process for anodically treating tin plated articles wherein work of widely varying quality withrespect to chemiduced within a pre-determined range of Wholly practical conditions.

Referring now more particularly to the practice of this invention, the tin plate will normally have Wide variations in the amount of tin on the surface, and will vary widely with respect to surface conditions, such as chemical composition, the presence of embedded organic foreign matter, porosity of the tin layer, and the crystalline structure of the tin surface. It is undesirable to have a heat reflecting exterior surface in a bread baking operation, and the conventional burning in process will reproduce oxides of tin corresponding to the initial surface condition.

The anodic chemical process of the present invention for treating tin surfaces will substantially reduce the heretofore mentioned objections; and in addition, will substantially remove objectionable carbonaceous deposits within the porous tinplate, such as grease and oil, which will ultimately cause a non-uniform oxide layer to be formed on the surface of tin by the conventional burning in process.

In the chemical anodic treatment of tin plate, the articles or products of various shapes and configurations are electrolytically treated by using one or more of the articles or products as the anodes in an electrolytic bath.

The electrolytic solution consists of an alkaline composition which generally may contain one or more water soluble alkaline compounds. These compounds may be defined generically as complexing reagents having basic or alkaline properties. These reagents generally are defined as a basic salt of an inorganic or organic acid or a base.

It has been found that satisfactory tin oxide coatings may be formed in an alkaline solution of a basic salt wherein such salt may be derived from inorganic or organic acids, such as chromic acid, silicic acid, phosphoric acid, fiuosilicic acid, boric acid, fiuoboric acid, citric acid, tartaric acid, glycollic acid, carbonic acid, oxalic acid, malonic acid and other similar generically related acids.

Under certain conditions, primarily using extremely high current densities, interference films of the tin oxide may be formed in substantially neutral solutions. These films do not exhibit the desired chemical and physical properties related to adhesion and heat absorption.

The scope of this invention shall not be limited to the specific chemical composition with respect to any of the complexing reagents.

In the anodic treatment of tin plated articles and products in the electrolyte, it has been found advantageous to maintain current densities ranging from about 4 amperes per square foot to about 100 amperes per square foot of tin surfaces undergoing treatment together with a solution temperature of at least 50 C.; and usually more, up to the maximum temperature which falls below the boiling point of the solution so as not to cause excessive evaporation. Under the chemical conditions specified together with time of immersion, it is possible to obtain a uniform meta-stable tin oxide coating on tin plate having thickness ranging from about 35 micro inches to almost complete conversion of the free tin surface.

It is preferred to employ an electrolyte which by weight consists of at least 0.5% up to about 40% of the complexing reagents wherein the pH of the solution may be adjusted if necessary, with the corresponding alkali or combination ofother alkalies so that the pH range may vary from about a pH of 7 to a pH of about 13; and the remaining parts needed to form l% by weight being substantiallyof Water. It has been found that the anodic stannocomplexes are more readily formed when the pH is within the range of about 7.5 to 11.

One or more other tin plated articles such as tin formed baking pans are made the anode of the electrolytic solution and are subject to anodic treatment while maintaining a preferred solution bath temperature of about 40 to 100 C. and a current density of about to 40 amperes per square foot. The cathode may consist of shaped metallic compositions, such as stainless steel or the lead lined tank may be used as the cathode. Under such temperature conditions, the solution is found to remain stable to the extent that the complexing reagent is substantially retained in the bath with respect to weight percentage.

A uniform meta-stable oxide of tin may be imparted to the tin surface in a short interval of time. The time of immersion together with solution conditions are impor nt factors with regard to the amount of meta stable tin oxide formed by the anodic treatment. it has been found that interference films of about 2 to 6 micro inches thick may be formed by anodic treatment in the electrolyte fer an interval of time corresponding to a few seconds. In addition, it has been found that in a matter of minutes the entire tin layer can be converted into a meta stable oxide of tin.

As examples of other exceptionally stable, conductive and highly effective electrolyte solutions, and the related operating conditions which are employed for rapidly obtaining a uniform continuous meta stable oxide of tin, mention is made of the following:

Treatment A Electrolyte Sodium Dichrornate (NazCrgOfi Sodium Carbonate (NngCOp Any remaining parts needed with the above to total 100 percent by weight, being substantially water.

Bath temperature 50 C. C.

pH range 8.5 to 12.5.

Current density 12-60 amp/sq. ft.

Time of immersion 30 to 190 seconds.

Treatment B Electrolyte Sodium Dichromate (NazGmO Sodium Tctrahorate (NILZB4O7) Any remaining parts needed with the above to total percent by weight, being substantially water. Bath temperature 50 C.30 C. pH range W 8.0l1.5.

Current density 1260 amp/sq. f t. Time of immersion 30 to seconds.

Treatment C Perot-n t. Electrolyte Total Weight. 1 of Bath l Sodium Dicllromatc (Na Cr201) s i c Q. ::--It Sodium Phosphate (Na lOl) tun-n5 Any remaining parts needed with the above to total 100 percent by Weight, being substantially water.

it so;

pH range 8.013.5. Current density 1260 amp/sq. ft. Bath temperature -50 C.80 C. Time of immersion 30 to 190 seconds.

TreatmentE Percent Electrolyte Total W'eight of Bath Sodium Tartrate (Na O4H4O .2H O) 3-30 Disodtum Hydrogen Phosphate (Na HPo 2-10 Any remaining parts needed with the above to total 100 percent by weight, being substantially water.

pH range 8.012.5. Current density 4-60 amp./ sq. ft. Bath temperature 90 C.-100 C. Time of immersion to 120 seconds. Treatment F Percent Electrolyte Total Weight of Bath Dlsodium Hydrogen Phosphate (N azHPo 4. 5-40 Any remaining parts needed with the above to total 100 percent by weight, being substantially Water. PH adjusted with phosphoric acid (H3PO4) or sodium hydroxide (NaOH) as required.

pH range 7.0-13.0 Current density 12-60 amp./ sq. ft. Bath temperature 80 C.-100 C. Time of immersion to 190 seconds.

Treatment G Percent Electrolyte Total Weight of Bath Dlsodium Hyldrogen Phosphate (N azHPOQ 3. 8-47 Trlsodlum P osphete (Na PO 1. 0-20 Any remaining parts needed with the above to total 100 percent by weight, being substantially water.

Any remaining parts needed with the above to total 100 percent by weight, being substantially water.

pHrange 7.0-l2.0. Current density 12-60 amp/sq. ft. Bath temperature 80 C.100 C. Time of immersion 30 to 190 seconds.

Sodium hydroxide: as required to adjust pH to specified value.

Treatment I Percent Electrolyte Total Weight of Bath Sodium Tartrate (NazOlHiomHzo) 6-40 6 Any remaining parts needed with the above to total percent by weight, being substantially water. Sodium hydroxide: as required to adjust pHspecified value.

pHrange 8.5-125.

Current density 12-60 amp/sq. ft. Bath temperature 70 C.-95 C. Time of immersion 30 to 190 seconds.

Particularly, an excellent meta stable oxide layer of tin was obtained on tin plate in the instance of Treatment G and Treatment H. The scope of the invention should not be bound by any such quality of chemical composition, nor by the specific proportions of alkali, salts and water given in the several illustrative examples of the treatment.

Furthermore, it shall be noted that by using Treatment H it has been found that over a pH range from about 9.5 to 10.5, the process is particularly adaptable to anodically treating the tin plate to form the meta stable oxide layer of tin. In addition, this particular treatment may be used wherein a fabricated baking pan set having galvanized iron strapping affixed thereto may be anodically treated, wherein the tin surface is converted to a meta stable form of the oxide of tin and the zinc galvanize surface is somewhat anodically treated as well. If the bright galvanize is desired, a conventional bright dip may be used consisting of chromic acid and nitric acid for brightening the zinc surface.

The term meta stable form of oxide of tin as used in this invention substantially denotes a mixture of stannic and stannous oxides wherein the blue-black meta stable form of a mixture of oxides of tin is meta stable at elevated temperatures thereby forming a substantially olive-green color stable oxide of tin.

The treated tin plate articles are then removed from the electrolyte bath and rinsed with water in order to remove any occluded salts. The treated articles are then dried at slightly elevated temperature in order to remove the adherent water. The treated tin plate articles upon which the meta stable oxide of tin is deposited thereon are then subjected to an oxide conversion temperature condition whereby the meta stable oxide of tin is then converted at elevated temperatures, from about C. to 230 C., to the stable form of the oxide of tin, primarily a mixture of stannic and stannous oxide of tin. A lowertemperature range may be used under certain oxidizing conditions, thereby increasing the time cycle for conversion. This step in the process may be obtained by direct heat application to formulate the oxide conversion, although exposure to air or to any oxidizing condition will somewhat accelerate the oxide conversion.

The formation of the anionic stanno-complex under electrolytic treatment will vary depending upon solution compositions and conditions. Since tin exhibits amphoteric properties, it has been noted that the meta stable oxide of tin may be formed under alkaline conditions as well as under certain acid conditions. In addition, it has been noted that the formation of the meta stable oxide of tin may be formed first by immersion in an alkaline media and then subsequently into an acid media or combinations thereof.

It shall be noted that various hydrates of the aforementioned salts may be used, singly or in combination with the salts. By the use of hydrates of various salts in the electrolyte the percent by weight of the various salt(s) would be increased and the relative percent by weight of water would be proportionately decreased.

The complexing reagent forms the anionic stanno-complex which in turn is converted into the hydrate of stannous-stannic oxide. The hydrate is unstable in form and is then partially converted into the meta stable form of stannous-stannic oxide. The meta stable stannousstannic oxide exhibits the interference films on the surface of tin, and when the thickness of the meta stable stannous-stannic oxide appears to approach seven (7) 11' micro inches, opacity of the film begins to occur. By varying the time of immersion in the electrolytic bath, the blue-black meta stable stannous-stannic oxide is formed and at about 2 to minutes immersion time under the aforementioned conditions it appears that the tin layer may be converted totally into the meta stable stannous-stannic oxide. Therefore, it is desirable to reduce the time of electrolytic immersion to about fifteen to ninety seconds in order to retain a relatively high percentage of free tin.

The time interval for electrolytic immersion will vary depending upon solution conditions. By reducing the current density and/ or the temperature as well as the concentration of the complexing reagent, it is possible to increase substantially the time interval. of immersion of the tin layer in the electrolytic bath. Therefore, the scope of the invention shall not be bound by the specific time interval of treatment specifically aforementioned.

The meta stable stannous-stannic oxide exhibits various transitional interference colors approaching the blueblack. The blue-black is then considered the end point of opacity and it has been found that the interference colors or red, purple, green and blue-green will upon subjecting the then treated meta stable stannous-stannic oxide to conversion conditions at elevated temperatures convert into a stable stannous-stannic oxide film structure exhibiting a uniform olive-green color.

In examining a. micro cross-section of tin plate that has been subjected to the anodic treatment and temperature conversion, we have found that the stannous-stannic oxide layer may be varied in thickness ranging from a few micro inches to a thickness equivalent to the entire layer of available free tin. Micro cross-section examination of a one and one-half pound per basis box of untreated tin plate has revealed a thickness of free tin plus the tiniron alloy layer of about 0.000090 inches. The tin-iron alloy layer on hot dipped tinplate will be approximately equivalent to about 0.000010 to 0.000015 inch thick. The amount of free tin on each surface of one and one-half pound tin plate per basis box will approximate from 0.000060 to 0.000080 inch thick.

By allowing a sufiicient time interval to elapse in the electrolytic immersion step, the free tin can be substantially converted into the meta stable form of stannousstannic oxide. This appears to be undesirable and chemical control of the amount of meta stable stannous-stannic oxide is imperative. It has been found that a tin oxide layer of approximately 0.000007 to 0.000030 inch thick will exhibit the preferred uniformity of color as to heat absorption on the surface of the tin plate thereby retaining approximately 0.00004 to 0.00007 inch of free tin. Experimentally, it has been found that excellent bread baking was produced by converting substantially all of the free tin to tin oxide; although the corrosion resistance of the surface is slightly decreased.

It has been found highly desirable, in order to extend the life of certain baking implements, to produce a lesser amount of oxide of tin on the inner surface of a tin plate blank or a baking pan. This result may be accomplished by masking the surface of the tin plate blank, or the baking pan, by placing two blanks face to face, or two baking pans face to face, so that there is a substantial reduction in the current density in solution with respect to the contiguous surfaces. This procedure may be improved by placing a non-conductive material. such as plastic materials produced from phenol-formaldehyde or methyl methacrylate compositions, in front or intermediate of the section wherein it is desired to reduce the efifective current density. This is substantially a masking operation.

Prior to the final temperature conversion step, the meta stable stannous-stannic oxide surface may be coated with a stable high temperature organicbreadreleasing, film, such as alkyl aryl silicones, polytetrafluoroethylene,, polytrifluoromonochloroethylene and other high polymer filmforming organic substances and/or mixtures thereof; and then subjected to the temperature conversion step wherein independently but simultaneously the meta stable stannousstannic oxide is converted to the stable stannous-stannic oxide and the organic film is cured. Excellent adhesion of the organic bread releasing films to the treated tin oxide films was obtained. Furthermore, certain substrate organic films, such as the epoxide resins, may be coated intermediate to the tin oxide coating and the aforementioned bread releasing films in order to enhance the adherence of the above-identified bread releasing films.

As many possible embodiments may be made of our invention and as many. changes may be made in'the .embodiments hereinbefore set'forth, it is to be understood that all matter described herein is to be interpreted-as illustrative and not as a limitation.

This application is a continuation-in-part of co-pending application Serial No. 150,352, filed March 17, 1950, issued August 31, 1954, as Patent No. 2,687,994.

We claim as our invention:

1. In the formation of a heat absorbing tin oxide coating on a tin surface article, the process which comprises subjecting the tin article to anodic electrolytic treatment in an aqueous alkaline electrolyte consisting of about 1.5% to 12% by Weight disodium hydrogen phosphate and about 1.5% to 12% by weight boric acid, passing current through said electrolyte to form an oxide of tin, removing the oxidized tin article from the electrolyte and then subjecting the anodized article to an elevated temperature from about C. to 230 C. to convert said oxide to green oxide of tin.

2. In the formation of a heat absorbing tin oxide coating on a tin surface article, the process which comprises subjecting the tin article to anodic electrolytic treatment in an aqueous alkaline electrolyte consisting of about 0.5% to 3.5% by weight sodium dichromate and about 1% to 5.5% by weight sodium carbonate, passing current through said electrolyte to form an oxide of tin, removing the oxidized tin article from the electrolyte and then subjecting the anodized article to an elevated temperature from about 180 C. to 230 C. to convert said oxide to green oxide of tin.

3. In the formation of a heat absorbing tin oxide coating on a tin surface article, the process which comprises subjecting the tin article to anodic electrolytic treatment in an aqueous alkaline electrolyte consisting of about 0.5 to 3% by weight sodium dichromate and 1.25% to 5.5 by weight sodium phosphate, passing current through said electrolyte to form an oxide of tin, removing the oxidized tin article from the electrolyte and then subjecting the anodized article to an elevated temperature from about 180 C. to 230 C. to convert said oxide to green oxide of tin.

4. In the formation of a heat absorbing tin oxide coating on a tin surface article, the process which comprises subjecting the tin article to anodic electrolytic treatment in an aqueous alkaline electrolyte consisting of about 0.5% to 3% by weight sodium dichrornate and 1% to 6% by weight sodium metasilicate, passing current through said electrolyte to form an oxide of tin, removing the oxidized tin article from the electrolyte and then subjecting the anodized article to an elevated temperature from about 180 C. to 230 C. to convert said oxide to green oxide of tin.

References Cited in the file of this patent UNITED STATES PATENTS 2,687,994 Russell et al. Aug. 31, 1954 

1. IN THE FORMATION OF A HEAT ABSORBING TIN OXIDE COATING ON A TIN SURFACE ARTICLE, THE PROCESS WHICH COMPRISES SUBJECTING THE TIN ARTICLE TO ANODIC ELECTROLYTIC TREATMENT IN AN AQUEOUS ALKALINE ELECTROLYTE CONSISTING OF ABOUT 1.5% TO 12% BY WEIGHT DISODIUM HYDROGEN PHOSPHATE AND ABOUT 1.5% TO 12% BY WEIGHT BORIC ACID PASSING CURRENT THROUGH SAID ELECTROLYTE TO FORM AN OXIDE OF TIN, REMOVING THE OXIDIZED TIN ARTICLE FROM THE ELCTROLYTE AND THEN SUBJECTING THE ANOXIZED ARTICLE TO AN ELEVATED TEMPERATURE FROM ABOUT 180* C. TO 230* C. TO CONVERT SAID OXIDE TO GREEN OXIDE OF TIN. 